1. Field of the Invention
The present invention relates to an image sensing apparatus which carries out focus adjustment by driving an image sensing optical system by using a video signal obtained from an image sensor.
2. Description of the Related Art
In the field of this kind of apparatus, a system has heretorefore been known which is arranged to extract a high-frequency component contained in a video signal by means of a band-pass filter or the like or extract the sharpness of an edge portion of the subject image by a video-signal differentiating process or the like, make a decision as to the state of a subject image on the image sensing plane of an image sensor, and attain an in-focus state by moving the position of a lens so that the extracted amount reaches a maximum. This system is described in detail in "Automatic Focus Adjustment of Television Camera by Hill Climbing Servo System" written by Ishida, et al. ("NHK Technical Research", 1965, Vol. 17, No. 1). Such system will be described in brief below with reference to FIGS. 1 and 2.
The example shown in FIG. 1 includes an image sensing lens 1 which constitutes one element of an image sensing optical system, an image sensor 2, a preamplifier 3, a process circuit 4, a band-pass filter (hereinafter called "BPF") 5, a detecting circuit 6, a motor driving circuit 7 and a motor 8.
A subject image, which is formed on an image sensing plane (focal plane) of the image sensor 2, is converted into an electrical signal by the image sensor 2, and the electrical signal is obtained as a video signal from the image sensor 2. The video signal is amplified to a suitable level by the preamplifier 3, and the output of the preamplifier 3 is converted into a standardized video signal, such as an NTSC television signal, by the process circuit 4. The output of the preamplifier 3 is also applied to the BPF 5, in which a high-frequency component contained in the video signal is extracted. The detecting circuit 6 provides an output equivalent to the absolute amount of the high-frequency component.
As shown in FIG. 2, the output of the detecting circuit 6 varies depending on the amount by which the image sensing lens 1 is moved forward or backward, and shows its maximum at a certain position. The reason for this is as follows. As the amount of movement of the image sensing lens 1 varies, the state of focus of the image projected on the image sensing plane of the image sensor 2 varies accordingly. In the case of an in-focus state, i.e., the state in which the projected image is in focus, the sharpness of the projected image reaches its maximum, and if the projected image of maximum sharpness is converted into a video signal by the image sensor 2, then the video signal contains the maximum absolute amount of high-frequency component.
The motor driving circuit 7 controls the direction of rotation of the motor 8 so that the image sensing lens 1 moves in the direction in which the output of the detecting circuit 6 becomes larger, and stops the motor 8 at a position where the output of the detecting circuit 6 reaches its maximum.
In the above-described manner, the amount of movement of the image sensing lens 1 of the image sensing apparatus is controlled to automatically attain the in-focus state.
In the above-described system, if it is assumed that the image sensing lens 1 is initially located at the position indicated by a point A of FIG. 2, it is necessary to temporarily move the image sensing lens 1 in the direction of an infinity end or a closest-distance end and determine the direction of driving of the image sensing lens 1, for the purpose of moving the image sensing lens 1 to a point B which is an in-focus position. For example, if the image sensing lens 1 is driven at the point A of FIG. 2 in the direction of the infinity end, it is only necessary to continue driving the image sensing lens 1 until the image sensing lens 1 reaches the point B. However, if the image sensing lens 1 is driven at the point A in the direction of the closest-distance end, it is necessary to reverse the driving direction of the image sensing lens 1 after it is confirmed that the output of the detecting circuit 6 has lowered.
Even if the image sensing lens 1 is being driven in the direction of the point B, since it is impossible to determine, upon arrival of the image sensing lens 1 at the point B, whether the output of the detecting circuit 6 obtained at the point B is the maximum, it is necessary to perform the operation of causing the image sensing lens 1 to pass the point B, confirming a lowering in the output of the detecting circuit 6 at a point C and returning the image sensing lens 1 to the point B.
The above-described operation must be neccessarily performed because if no variation of the output of the detecting circuit 6 is confirmed while driving the image sensing lens 1, it is impossible to determine whether the subject image is in focus, out of focus with an image point of the subject image being deviated in the direction of an infinity end (hereinafter referred to as "far focus"), or out of focus with the image point of the subject image being deviated in the direction of a closest-distance end (hereinafter referred to as "near focus"). For this reason, the above-described operation is not suitable for the purpose of attaining an in-focus state automatically, efficiently and smoothly.
To overcome the disadvantages of the above-described system, a system is proposed which is arranged to make a decision as to the state of focus of a subject image in an image sensing plane by periodically vibrating part of a lens system or an image sensor along the optical axis by a small degree at specific intervals, thereby automatically attaining an in-focus state. Such a system is described in detail, for example, in Japanese Laid-Open Patent Application No. Sho 58-188965 (Kitamura, et al.). According to this system, since it is possible to determine whether the state of focus is in focus, near focus or far focus without driving the lens system, it is possible to effect a focusin operation efficiently and smoothly as compared with the previously-described system.
However, the latter conventional example requires a complicated and expensive mechanism for vibrating the lens system or the image sensor along the optical axis. Further, if the state of focus is deviated from an in-focus state to an excessive extent, a large magnitude of vibration must be produced with the result that a subject image is unnaturally photographed as if it were vibrating in itself.